Practice Essentials

Thallium is a heavy metal used in the manufacture of electronic components, optical lenses, semiconductor materials, alloys, gamma radiation detection equipment, imitation jewelry, artist's paints, low temperature thermometers, and green fireworks.
[1, 2, 3] Trace amounts of thallium are used as a contrast agent in the visualization of cardiac function and tumors. Thallium exposure may occur at smelters in the maintenance and cleaning of ducts and flues and through contamination of cocaine, heroin, and herbal products. Criminal and unintentional thallium poisonings are still reported, some leading to death.
[4, 5]

Background

Thallium is a heavy metal that was serendipitously discovered by Sir William Crookes in 1861 while trying to extract selenium from the by-products of sulfuric acid production. Crookes named the new element "thallium" from the Greek thallos, meaning "green shoot or twig" after the bright green spectral emission lines that identified the element. In 1862, Claude-Auguste Lamy independently isolated thallium, studying both its chemical and physical properties.
[9]

In the past, thallium was used as a therapeutic agent to treat syphilis, gonorrhea, tuberculosis, and ringworm, and it was also used as a depilatory for excess hair. In the early part of the last century, a product known as Koremlu (thallium acetate) was marketed in the United States for the treatment of ringworm as well as a depilatory agent. By 1934, 692 cases of thallium poisoning were reported with at least 31 deaths.
[4, 5] Thallium was also widely used as a rodenticide. Its use as a household rodenticide was banned in the United States in 1965 after multiple unintentional poisonings.
[10] Commercial use was banned a decade later. Unfortunately, unintentional poisonings are still reported in other countries where thallium is used as a rodenticide and ant killer.

Thallium is a soft and pliable metal. It melts at 303.5°C and boils at 1482°C. It is colorless, odorless, and tasteless. Thallium has a similar ionic radii to potassium (Tl 0.147 nm vs K 0.133 nm), which is one principle behind its toxicity.
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Pathophysiology

The biochemical research on the cellular effects of thallium is extensive, but few data exist in humans. The structural similarity of thallium to potassium results in the body treating it as such—an action that is key in poisoning.
[11] Thallium demonstrates at least the following 5 major toxicologic effects
[12] :

Disruption of potassium-dependent processes

Riboflavin sequestration

Interference with cysteine residues

Ribosomal inhibition

Myelin sheath injury

Thallium accumulates in tissues with high potassium concentrations such as muscle, heart, and central and peripheral nerve tissue. Thallium’s similar size to potassium results in early stimulation then inhibition of potassium-dependent processes. Key enzymes involved in thallium toxicity include pyruvate kinase and succinate dehydrogenase. Their inhibition leads to impaired glucose metabolism and disrupts the Kreb’s cycle leading to decreased ATP production. In addition, sodium-potassium ATPase is affected, resulting in cell membrane injury. This enzymatic injury results in swelling and vacuolization of mitochondrial and cell death. Within the mitochondria, thallium also causes sequestration of riboflavin resulting in the inhibition of flavin coenzyme flavin adenine dinucleotide (FAD), impairing the electron transport chain, and further reduction of ATP.

Similar to other metals, thallium has a high affinity of disulfide bonds. This interferes with cysteine residue cross-linking reducing keratin formation. This results in alopecia and the formation of Mees lines. Decreased cysteine cross-linking also leads to decreased glutathione resulting in accumulation of lipid peroxides in the brain, which are most prominent in the cerebellum, often seen as dark pigmented lipofuscinlike areas.
[13]

Thallium interferes with protein synthesis by damaging ribosomes, particularly the 60s ribosome, further leading to cellular injury and death.
[14]

Although the exact mechanism of myelin injury by thallium is unknown, there are consistent findings of fragmentation and degeneration of myelin in both the central and peripheral nervous systems. A Wallerian degeneration pattern first develops in long peripheral axons (lower then upper extremities) with sensory then motor impairment.

Thallium follows a 3-phase toxicokinetics: first intravascular distribution, then CNS distribution, and finally elimination. In the first 4 hours following exposure, thallium is rapidly distributed to the blood and to well-perfused organs such as the kidney, liver, and muscle. Over the next 4-48 hours, thallium is distributed into the CNS. The elimination phase begins about 24 hours after ingestion.

Thallium is primarily eliminated through excretion into the feces (51.4%) and the urine (26.4%). The high concentrations of thallium found in the kidney (>5.5 times more than other tissues) result from renal filtration with approximately 50% reabsorbed in the kidney tubules. Elimination is slow with an elimination half-life of 3-30 days, varying with the dose and chronicity of the exposure. Because of this prolonged elimination phase, thallium may act as a cumulative poison.

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Epidemiology

Frequency

United States

In 2016, 29 single exposures with one major outcome and no deaths were reported by the American Association of Poison Control Centers' National Poison Data System (AAPCC-NPDS). Most involved adults, and none of the exposures were intentional.
[15]

International

Thallium toxicity is likely more common in developing countries where thallium rodenticides are still in use, but few data exist as to the incidence of thallium poisoning outside the United States.
[16, 8]

Mortality/Morbidity

The mortality rate for acute thallium toxicity has been reported as 6-15%; among survivors, 33-50% have neurologic or ocular sequelae.

The lethal dose for humans is 10-15 mg/kg (around 1 g for a 70-kg person). However, death can still occur at lower doses (minimal reported dose was 8 mg/kg). Some treated patients have survived exposure up to 28 mg/kg.

Race-, Sex-, and Age-related Demographics

No scientific data substantiate any differences in thallium toxicity that are attributable to race, sex, or age.

Igor Boyarsky, DO is a member of the following medical societies: American Academy of Anti-Aging Medicine, American Academy of Emergency Medicine, American College of Emergency Physicians, and American Osteopathic Assocation